Many diseases could be treated in a more physiologic fashion if tissue from lower animals could be transplanted into humans. Immunoisolation, as its name implies, is the protection of transplanted organs, tissues, cells, etc. from attack by the host immune system. Isolation from the host immune system is accomplished by the use of a semipermeable membrane.
Recent work by Brauker et al. has demonstrated that a prescribed membrane architecture can promote vascular structures near the host tissue-membrane interface. Such membranes had pores that were formed by membrane structures (strands or fibers) with a diameter of less than 5 .mu.m, whereas membranes that did not develop near vascular structures had cavities with "plate-like" qualities, having diameters greater than 5 .mu.m. Histological examination of the vascularizing membranes revealed that the invading inflammatory cells (of the host) had a rounded morphology, while the cells were flattened in the membranes that did not have close vascular structures. See, Brauker et al., Neovascularization of Synthetic Membranes Directed by Membrane Microarchitecture, J. Biomed. Mat. Res., In Press; Brauker, J., Martinson, L., Young, S., Johnson, R. C.: Neovascularization at a membrane-tissue interface is dependent on microarchitecture. Transactions of the Fourth World Biomaterials Congress Apr. 24-28, 1992 p. 685; Brauker, J., Martinson, L., Carr-Brendel, V. E., and Johnson, R. C.: Neovascularization of a PTFE membrane for use as the outer layer of an immunoisolation device. Transactions of the Fourth World Biomaterials Congress Apr. 24-28, 1992 p. 676; Brauker, J., Martinson, L., Hill, R., and Young, S.: Neovascularization of immunoisolation membranes: The effect of membrane architecture and encapsulated tissue. Transplantation 1:163,1992; and Brauker, J., Martinson, L. A., Hill, R. S., Young, S. K., Carr-Brendel, V. E., and Johnson, R. C.: Neovascularization of immunoisolation membranes: The effect of membrane architecture and encapsulated tissue. Transplantation Proceedings 24:2924, 1992; copending Brauker et al. U.S. patent application Ser. No. 08/210,068, filed Mar. 17, 1994, entitled "Close Vascularization Implant Material."
Membranes of the type characterized by Brauker et al. facilitate high levels of vascularization near the membrane-host tissue interface.
Vascularization-promoting membranes of the type characterized by Brauker at el. still must be used in association with an immunoisolation membrane. The immunoisolation membrane is placed between the vascularization membrane and the implanted tissue and has a pore size sufficient to block penetration by host vascular structures completely through the permeable boundary that separates the implanted cells from host tissue. Such penetration breaches the integrity of the boundary , exposing the implanted cells to the complete immune response of the host. Furthermore, the immunoisolation membrane must also prevent the passage of host inflammatory cells (in the case of allografts) and the passage of both host inflammatory cells and molecular immunogenic cells (in the case of xenografts). Vascularization-promoting membranes of the type characterized by Brauker et al., when used in association with an immunoisolation membrane, are capable of supporting allografts at high tissue densities for extended periods, even in the absence of immunosuppressive drugs.
The need for both a vascularization-promoting membrane and an immunoisolation membrane has resulted in laminated membrane structures. For example, Clarke et al U.S. Pat. No. 5,344,454 discloses the lamination of a GORE-TEX.TM. membrane material (which serves as a vascularization-promoting membrane) and a BIOPORE.TM. membrane material (which serves as an immunoisolation membrane for allografts) using a criss-crossing pattern of nonpermeable polymeric adhesive. It has been observed that this laminated structure can experience delamination when implanted. This delamination is caused by the infiltration of host inflammatory cells between the two membrane materials, forcing the material apart. This delamination can increase the diffusion distance the laminated membrane structure presents between the host vascular structures and implanted tissue. This, in turn, can adversely effect the passage of nutrients through the laminated membrane structure to the implanted cells.